1. Field of the Invention
The present invention relates to a tempo detection apparatus, a chord-name detection apparatus, and programs for these apparatuses.
2. Discussion of Background
In a conventional automatic musical accompaniment apparatus, the user specifies a tempo of performance in advance and automatic accompaniment is conducted according to the tempo. When a player gives a performance with this automatic accompaniment, the player needs to play according to the tempo of the automatic accompaniment. It is very difficult especially for a novice player to perform in that way. Therefore, an automatic accompaniment apparatus has been demanded which automatically detects the tempo of the performance of a player from the sound of the performance and performs automatic accompaniment according to the tempo.
In a music-transcription apparatus for detecting chords and musical-notation information from a sound source such as a music CD containing recorded performance sound, a function of detecting the tempo from the performance sound is required as a process in a stage prior to transcribing a melody.
One such tempo detection apparatus is disclosed, for example, in Japanese Patent No. 3,231,482.
This tempo detection apparatus includes tempo change section which detects, based on performance information indicating the tone, sound volume and sound timing of each note in externally input performance sound, an accent caused by the sound volume and an accent caused by a musical factor other than the sound volume. The tempo change means predicts change of tempo based on performance information according to these two accents, and adjusts an internally produced tempo to follow the predicted tempo. Therefore, it is necessary to detect musical-notation information in order to detect the tempo. When a musical instrument such as a MIDI device having a function to output musical-notation information, is used for performance, musical-notation information can be obtained easily. However, if an ordinary musical instrument not having such a function is used for performance, a music transcription technique for detecting musical notation information from the performance sound is required.
One tempo detection apparatus that receives performance sound, that is, an acoustic signal, of an ordinary musical instrument having no function for outputting musical-notation information, is disclosed, for example, in Japanese Patent No. 3,127,406.
In this tempo detection apparatus, an input acoustic signal is subjected to digital filtering in a time-division manner to extract chromatic notes, the generation period of the detected chromatic notes is detected from the envelop value of the note, and the tempo is detected according to the meter of the input acoustic signal, specified in advance, and the generation period of note. Since this tempo detection apparatus does not detect musical-notation information, the apparatus can be used in a pre-process of a music transcription apparatus which detects chords and musical-notation information.
A similar tempo detection apparatus is also described in “Real-time Beat Tracking System”, Masataka Goto, Computer Science Magazine Bit, Vol. 28, No. 3, Kyoritsu Shuppann, 1996.
Chords are a very important factor in popular music. When a small band plays a popular music, they usually use a musical score called a chord score or a lead sheet having only a melody and a chord progression, not a musical score having musical notation to be played. Therefore, to play a musical piece such as that in a commercial CD with a band, it is necessary to transcribe the performance sound into chord progression of the musical piece. This work can be performed only by professionals having special musical knowledge and cannot be performed by ordinary people. Consequently, there have been demands for an automatic music transcription apparatus which detects chords from a musical acoustic signal with the use of e.g. a commercial personal computer.
Such an apparatus for detecting chords from a musical acoustic signal is disclosed in Japanese Patent No. 2,876,861. This apparatus extracts, candidates of fundamental-frequencies from a result of power-spectrum calculation, removes what seem to be harmonics from the candidates of fundamental-frequencies to detect musical-notation information, and detects the chords from this musical-notation information.
However, it has been known that it is very difficult for this apparatus to remove the harmonics because of difference of harmonic structure due to the difference of the types of musical instruments, difference of harmonic output due to the difference of key-hitting strength, changes of the power of harmonics with time, phase interference among notes having the same frequencies as harmonics, and others. In other words, it is not likely that the process for detecting musical-notation information always works correctly for sound sources such as general music CDs containing a mixture of songs and sounds of many musical instruments.
A similar apparatus for detecting chords from a musical acoustic signal is disclosed in Japanese Patent No. 3,156,299. This apparatus applies to an input acoustic signal digital filtering processes of different characteristics in a time-division manner to detect the level of each chromatic note, sums up the detected levels of chromatic notes having the same scale relationships in one octave, and detects the chords by using a predetermined number of chromatic notes having larger summed-up levels. Since each piece of musical-notation information included in the acoustic signal is not detected in this method, the problem occurring in the apparatus disclosed in Japanese Patent No. 2,876,861 does not occur.